Polyethylene terephthalate (PET) and other types of plastic containers are commonly produced utilizing a machine referred to as a reheat, stretch and blow molder. The blow molder receives preforms and outputs containers. When a preform is received into a blow molder, it is initially heated and placed into a mold. A rod stretches the preform while air is being blown into the preform causing it to stretch axially and circumferentially, and take the shape of the mold. A typical reheat, stretch and blow molder has between ten (10) and forty-eight (48) or more molds. This increases the product rate of the blow molder, but also increases the rate at which defective containers can be generated when there is a problem with one or more blow molding process parameters. Accordingly, container manufacturers are keen to detect and correct blow molding process problems as efficiently as possible.
In the course of manufacturing blow-molded containers, it is desirable to control the blow molder to achieve desired container properties including, desired container dimensions, material distribution, strength, the absence of defects, etc. This is typically accomplished manually. According to one common technique, an operator of the blow molder ejects a set of completed containers for off-line inspection. Various types of off-line inspections are used to measure different aspects of the container. Material or thickness distribution is often measured using a qualitative “squeeze” test and/or a quantitative section weight test. In a squeeze test, the operator, or other testing personnel, squeezes the container to obtain a qualitative indication of whether there is sufficient material at key locations of the container. In a section weight test, the container is physically divided into circumferential sections. Each section is individually weighted, yielding the section weights. Other common off-line inspections include top load and burst pressure tests to measure container strength, volumetric fill height and base clearance tests to measure container size and shape, etc. Based on the qualitative and quantitative results of tests such as these, the operator modifies input parameters of the blow molder to move material to the appropriate locations within the bottle.
Manual measurement and adjustment techniques, however, suffer from several disadvantages. The qualitative nature of some of the tests makes it difficult to maintain consistent results from one tester to another. For example, different operators may interpret the same material distribution differently during a squeeze test. Also, even when using a rig, it is difficult to precisely replicate section cuts across multiple container samples, reducing the accuracy of manually obtained section weights. Further, it is very difficult for operators to consistently tune the blow molder to obtain the desired material distribution. The correlations between blow molder input parameters and output material distribution are very complex. Different blow molder operators have differing levels of understanding of these parameters and, therefore, differing abilities to obtain desired container distributions. As a result, many operators simply avoid modifying blow molder input parameters unless the containers are outside of design tolerances, even if an alternative material distribution would be desirable.
Early on-line inspection systems, such as the Intellispec™ product, available from Pressco Technology Inc. of Cleveland Ohio and the PET-View product, available from the Krones Group of Neutraubling, Germany, utilize computer vision to inspect containers either in or downstream of the blow molder and reject mal-formed containers. These systems improve the quality of the container production by removing containers with randomly occurring damage, inclusions, and grossly formed containers, but have limited success addressing process related issues that drive container quality and performance.
Subsequent inspection devices have allowed more detailed inspections to detect more subtle system properties. For example, the various infrared absorption measurement devices available from AGR International of Butler, Pa., are capable of measuring the material distribution of individual containers. The measurements are made using a series of emitters and sensors that are located either within or downstream of the blow molder. The sensors are oriented towards the sidewalls of the containers and generate measurements on the containers at 12.5 mm intervals, thus providing a profile of material distribution in the container sidewalls. Measurements from devices such as the AGR infrared absorption measurement devices obviate the need to conduct squeeze and section weight tests while, at the same time, providing more repeatable results. Also, advanced vision systems, such as the Pilot Vision™ system, also available from AGR International, Inc. of Butler, Pa., provide increased resolution and are able to detect more subtle container defects.
Recent advances in blow molder technology have allowed for some degree of automated process control in blow molders. For example, many current blow molders have mechanical mold controls that may be operated utilizing servo motors and other smart technology. Oven designs and control improvements have also improved. Also, recently the Sidel S.A.S. Company of Le Havre, France, has introduced a blow molder with a mold control loop that to accommodate variations in the temperature of performs arriving at the mold. The mold control loop controls the pre-blow start and pre-blow pressure to detect changes in preform properties and adapts the pre-blow pressure profile to account for any variations in preform energy or energy distribution.
Another process control system is the Process Pilot® product, available from AGR International, Inc. of Butler, Pa. The Process Pilot® product is a closed loop control system used to manage the re-heat stretch and blow molding process. An infrared absorption-type measurement system is used to generate a material distribution profile, as described above. The Process Pilot® product learns the relationship between the container blowing process and the location of the material in the container with a series of automated measurements made in conjunction with adjustments to the blow molder input parameters. This information forms the basis for future adjustments to the blow molder. A custom equation is used to express the relationship between blow molder input parameters and resulting material distributions. A control loop is implemented by establishing a baseline material distribution and baseline values for the various blow molder inputs. As the material distribution drifts during the blow molding process, relationship between the blow molder input parameters and container characteristics is utilized in conjunction with additional mathematics to determine blow molder parameter values that minimize the difference between the baseline and the measured material distribution while also minimizing control changes relative to baseline blow molder input parameters. The Process Pilot® product can be operated continuously to minimize the overall process variation.
Current blow molder process control systems represent an improvement over the prior off-line and often manual methods. Additional challenges remain, however. For example, current control systems described above do not consider container properties such as crystallinity or material density, base sag, various container dimensions, etc. These and other properties, which can have a significant effect on overall container quality, must still be managed with manual off-line tests and manual adjustments to blow molder input parameters. Improved process control systems for blow molders are needed.